The use of wood fibers, especially fibers from waste wood, is well known in the formation of articles such as oriented strand board (OSB) and particleboard. An OSB product is disclosed in U.S. Pat. No. 3,164,511 to Elmendorf, which suggests binding strands of wood with urea, phenol and melamine resin. One drawback of the process used to produce OSB products, however, is the high pressures required to form the OSB product. The OSB product, while suited for planar sheets for use as decking, is not ideally suited for use in the formation of articles having deep drawn or formed portions. Additionally, the wood fiber or strand thickness and length ranges permitted in OSB products render this product unsuitable for articles in which stresses must be uniformly transmitted throughout its structural matrix. The variations in fiber thickness and length of OSB creates high pressure and low pressure areas unsuitable for articles subjected to bending stresses. Further, OSB technology, as disclosed in U.S. Pat. No. 3,164,511, discourages the use of strands shorter than one-half inch or longer than 6 inches. Particleboard, formed similarly to OSB but using wood fines rather than strands as its main structural component, is very dense and brittle, and also requires very high pressure in its formation. The structural limitations associated with particleboard render it unsuitable for shaped, non-planar articles intended to withstand significant load stresses.
Other methods have been developed to utilize wood fiber in making shaped articles having drawn portions, such as pallets used in material handling, rather than planar articles such as decking. Such pallets normally include a flat support surface with either shallow or deep drawn legs projecting downwardly therefrom. Various such methods are known, and disclosed, for example, in the following U.S. Pat. Nos. 4,221,751, 4,248,163, 4,440,708, 4,960,553, and 5,142,994. These references disclose using either papermill sludge, the solid residue of wood fibers and filler, such as clay or wood fibers from other sources. The wood fibers disclosed in these references typically are bonded using thermosetting resins such as phenolformaldehyde, resorcinol-formaldehyde, melamine-formaldehyde, urea-formaldehyde, urea-furfural and condensed furfuryl alcohol resin and organic polyisocyanates, either alone or in combination. The use of isocyanates as the bonding agent in these articles normally prevents the mixture of wood fibers having different densities as the main structural components, since such bonding agents are very density dependent when used in such a process. This is an acute disadvantage in a system that must use waste wood from many different sources as a principal structural component. The use of these binding agents also has drawbacks such as contributing to the environmental waste stream, forming shaped products which are susceptible to moisture degradation, and forming products which do not have uniform strength characteristics throughout their load bearing portions.
Molded pallets and platforms also have been developed which, to some extent, utilize plastic, either as a coating over a cellulosic fiber matrix, or as an additive to wood pulp slurry, such as disclosed in U.S. Pat. Nos. 3,187,691 and 4,230,049. While the products produced thereby may achieve better moisture resistance characteristics than molded articles using other binding agents, such articles nevertheless suffer either from the strength limitations referenced above, or require an intricate and complex forming process. In some situations, plastic is added only as a coating in the final forming stages of the article, and thus does not impart significant bonding, strength, and other desirable characteristics achieved when plastic is used as a primary structural component as well as a bonding agent.
Accordingly, it is believed that a need yet remains for a method of forming a shaped article with specific and selective strength characteristics in which stresses on the article are transmitted substantially throughout the mat matrix, and in which the article includes additional characteristics such as ease in formation, ability to be formed into deep drawn shapes while maintaining its strength characteristics, and being resistant to degradation caused by moisture, insects and microbes.
It is also believed that a need remains for such a method which readily can utilize waste products, thus achieving the compound effect of reducing waste handling and landfilling, protecting the environment from waste incineration, and reinjecting used waste products back into a useful product stream. For example, while there is a trend toward recycling plastics, such recycling efforts are severely limited considering that most plastics of different composition cannot be combined to form a higher quality recycled product. Thus the potential known uses for recycled plastic as the main structural component for high grade finished products are relatively limited.
Additionally, the known methods which utilize wood fibers in some capacity to form a finished composite article do not adequately utilize the waste wood supply, which presently is either burned or landfilled. Many of the known methods utilize either papermill sludge as a source for wood fibers or are dependent upon a raw wood supply which is consistent in the same type of wood utilized. That is, known methods do not readily accept waste wood of various types having various densities in the same process due to the bonding agent limitations and structural inconsistencies discussed above.
The present invention overcomes many of these disadvantages by providing a method in which waste wood of varying types can be incorporated into the same product, thus providing for much better utilization of waste wood. Additionally, the present invention incorporates various types of plastics not only as a primary bonding agent but also as a primary structural component, thus also utilizing different types of plastics, which normally are not combinable for molding into a recyclable product. Even the articles manufactured according to the preferred embodiments of the invention can be recycled according to the methods disclosed herein to form "secondary products," so that the raw materials can be reused many times to form useful articles.